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PC Mode

StarPilot's Precompute (PC) function is used for predicting sextant heights for sight preparation, body ID, or for analyzing a series of existing sights of the same body — computing the theoretical slope of a series of sights is often a valuable aid to choosing the best sight among a scattered set. These operations do not require high precsion output, so when we must do this for moon or planets with a hand-help calculator, it is convenient to have a way to compute these data quickly. In most hand-held devices these are very slow computations, but StarPilot offers an option on this with the PC mode selection. PC mode equal "Normal" is used for quick computations in planning work, whereas PC mode equal High will yield the same precision we get in sight reductions. If you want just almanac data from the StarPilot, then use PC mode = High and do Precompute, but for extended moon or planet planning, use PC mode = Normal to save about half the computation time.

Notes on StarPilot Amanac Precisions
StarPilot is unique among celestial programs (PCs and hand-held calculators) in that it includes different modes for computing the astronomical data needed for celestial navigation. This allows us to use a Fast mode to calculate the entire sky quickly for planning or star id, and at the same time lets us do High precision sights of difficult bodies like Saturn and the Moon. We also inlcude a "Normal" mode which is used in the Precompute function and in the Sunrise/Sunset function.

The StarPilot "High" mode is the precision level that matches or surpasses the Nautical Almanac.... our theoretical specs are just slightly better. We automatically use this mode for all sight reductions. There is no user option on this — on the earliest versions of StarPilot this was a user option, but feedback from users led us to conclude that this was not the best arrangement.

The "Fast" mode is used internally for the Sight Planner, Star-Planet ID, and Moonrise/moonset computations. "Normal" mode is used for Sunrise/sunset and as an option in Precompute.

Specifics and references are listed here:

Body

Fast

Normal

High

Sun

Good-Med [A]

10" [B]

2" [C]

Moon

Low [A]

Low [N]

10" [D]

Mer, Ven, Mar

Med [E]

Good [F]

10" [C]

Jup, Sat

Med, Low [H]

Good, Low [I]

10" [J]

173 Stars

10" [L]

2" [M]

2" [M]
  1. Keplerian orbit using planetary elements for ecliptic circa 1990. Accuracy is approximately 10" of arc and increasing in error as one departs in time from the epoch. (Ref: 9)
  2. Keplerian orbits using planetary elements for ecliptic of date. Corrections for nutation and aberration are also included. (Ref: 7,9).
  3. Planetary programs are used to analytically compute positions. Corrections for nutation and aberration are also included. (Ref: 10, 9)
  4. ELP-2000/82 Lunar theory is used in addition to corrections for nutation and aberration. (Ref: 7,9)
  5. Keplerian orbits are computed using planetary elements for ecliptic of date. (Ref: 9)
  6. As in B with corrections for planetary aberration. (Ref: 7,9)
  8. As in E with additional corrections for Saturn and Jupiter. (Ref: 9)
  9. As in F with additional corrections for Saturn and Jupiter. (Ref: 7,9)
  10. Planetary tables are used for the period 1960-2100 AD. For years not in the stated period VSOP 87 planetary theory is used. Corrections for nutation and aberration are also included. (Ref: 10,7,9)
  12. Star positions for the ecliptic of 1975 are transformed to the positions relative to the ecliptic of date. (Ref: 3,9)
  13. As in L with additional corrections for nutation and aberration. (Ref: 3,9)
  14. As in A with corrections for nutation and aberration. (Ref: 3,9)

Note: All precision modes are acceptable for the computation of positions at sea with the possible exception of those listed with "low" precision. Modes listed as "low" may cause errors to intercept of up to several miles. Medium precision algorithms result in errors to intercept of less than .5 miles. Numeric values for maximum errors in precision are in seconds of arc. Please refer the listed references for further discussions on the accuracy of the methods used.

————

  1. Bibliography
  1. Nautical Almanac, US Naval Observatory, Washington DC, 1978,1981,1988,1991,1996.
  2. Almanac for Computers, US Naval Observatory, Washington DC, 1985.
  3. Astronomical Almanac, US Naval Observatory, Washington DC, 1975.
  4. Astronavigation Now it's Child's Play, Mike Harris, Practical Boat Owner, No 258, pg. 58, 1988.
  5. Which Star?, John Jeffrey, Practical Boat Owner, No 273, pg. 145, 1989.
  6. Practical Ephemeris Calculations, Oliver Montenbruck, Springer-Verlag Heidelberg, 1989.
  7. Astronomy on the Personal Computer, Oliver Montenbruck, Springer-Verlag Heidelberg, 1989.
  8. Astronomical Algorithms, Jean Meeus, Willman-Bell, Inc., 1991.
  9. Astronomical Formulae for Calculators, Willman-Bell, Inc., 1988.
  10. Practical Astronomy with Your Calculator, Peter Duffett-Smith, Cambridge University Press, 1988.
  11. Planetary Programs and Tables from -4000 to +2800, Pierre Bretagnon and Jean-Louis Simon, Willman-Bell, Inc., 1986.
  12. Celestial Navigation in the Computer Age, Alton B. Moody, Van Nostrand Reinhold Company, 1982.
  13. Navigation Afloat, Alton B. Moody, Van Nostrand Reinhold Company, 1980.
  14. Piloting/Navigation with the Pocket Calculator, Jack Buchanek and Ed Bergin, Tab Books, 1976.
  15. Coastwise Navigation, Stafford Campbell, Ziff-Davis Publishing Company, 1979.
  16. On the Overdetermined Celestial Fix, Thomas R. Metcalf and Frederic T. Metcalf, Navigation: Journal of the Institute of Navigation, Vol. 38, No 1, 1991.
  17. Advancing Celestial Circles of Position, Thomas R. Metcalf, Navigation: Journal of the Institute of Navigation, Vol. 38, No 3 1991.
  18. Manual for the Tamaya NC-77 Digital Navigation Computer, Tamaya Technics, Inc., Tokyo, Japan, 1978.

If you wish to learn still more about almanac computations, see the Positional Astronomy link in our Online Resources section.